1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Exercise about mol

  1. Oct 18, 2012 #1
    Hi everybody,

    sorry for my sloppy english. I have to translate the task so if you don't understand something just tell me and I try to rephrase it.

    1. The problem statement, all variables and given/known data

    The exersice is about the "Knallgas" equation. Two reaction euqations are given:

    [tex] 2H + O \xrightarrow \ 2H_2O +\Delta E[/tex]
    [tex] 2H_{2} + O_{2} \xrightarrow \ 2H_{2}O +2\Delta E[/tex]

    The energy which is released in this equation is: dE=285.8kJ

    a) Decide which reaction equation is the right one. The decision should be based on experimental obseravtions.

    b) How much grams oxygen is needed for an optimal reaction when I have 0.0038 cm³ hydrogen?

    c) How much energy is released? How long could someone use that energy to run a hairdryer with it (P=1,2 kW)

    2. Relevant equations


    3. The attempt at a solution

    a) Well, I think I can discuss that problem using the theorem from Avogrado. The ratio is the same in both equations. But in my opinion the task is missing a value for the volume of the water. But I say that the 2nd equation is right (That's what someone would find in the experiment)

    b) I searched for the density of H2

    [tex] \rho \cdot V=m [/tex]

    I get m=3.42*10^-4 kg
    [tex] \frac {m(H_2)} {m(O_2)}= \frac 1 8[/tex]

    Now I just have to multiply the mass of H2 times 8 in order to get the mass from O2. I get m(O2)=2.73*10^-3kg

    c) I use the law of conservation of mass. The whole mass of water equals m(H2)+m(O2). I searched for the molar mass of water in order to calculate how much mol I have. Afterwards I calculate the energy the following way:

    E= E(given)*mol

    and for the last task I used the formula: P=dE/dt

    Is my solution right? Are there other possibilities to solve it? I'm uncertain because I had to look up some stuff like molecular weight, density etc. But I don't know if there's another way

    Thank you for your help in advance
  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted